Based on the appearance thereof, a secondary battery may be generally classified as a cylindrical battery, a prismatic battery, or a pouch-shaped battery. A recent trend in the miniaturization of mobile devices has increased the demand for a prismatic battery or a pouch-shaped battery, which has a small thickness. An exploded perspective view of a pouch-shaped battery and a perspective view of an assembled state of the pouch-shaped battery are typically shown in FIGS. 1 and 2, respectively.
Referring to FIGS. 1 and 2, a pouch-shaped battery 10 includes a pouch-shaped case 20 having an inner space 21 of a predetermined size, a cover 30 hingedly connected to the pouch-shaped case 20, an electrode assembly 40 mounted in a receiving part 21 of the pouch-shaped case 20, the electrode assembly 40 including cathode plates 41, anode plates 42, and separators 43, electrode tabs 41a and 42a extending ends of the cathode plates 41 and the anode plates 42 of the electrode assembly 40, respectively, and electrode terminals 50 and 51 connected to the electrode tabs 41a and 42a, respectively.
A side extension part 22 of a predetermined width for thermal bonding is formed at an upper edge of the receiving part 21 of the pouch-shaped case 20. Middle portions of the electrode terminals 50 and 51 respectively connected to the electrode tabs 41a and 42a are coated with terminal tapes 52 made of an insulative material to prevent the occurrence of a short circuit between the electrode terminals 50 and 51 when the side extension part 22 of the pouch-shaped case 20 is thermally bonded to a side part 31 of the cover 30 using a thermal bonding device (not shown).
The conventional pouch-shaped battery with the above-stated construction is manufactured as follows.
First, the electrode assembly 40 including the cathode plates 41, the anode plates 42, and the separators 43 is mounted in the receiving part 21 of the pouch-shaped case 20 and then a predetermined amount of an electrolyte is injected into the inner space of the pouch-shaped case 20. At this time, the electrode tabs 41a and 42a of the electrode assembly 40 are respectively connected to the electrode terminals 50 and 51, the middle portions of which are coated with the terminal tapes 52. The electrode terminals 50 and 51 and the terminal tapes 52 partially protrude outward from the pouch-shaped case 20 and the cover 30.
Subsequently, the cover 30 is brought into tight contact with the pouch-shaped case 20 and then the side extension part 22 of the pouch-shaped case 20 is thermally bonded to the side part 31 of the cover 30 using the thermal bonding device (not shown) such that the electrolyte does not leak from the pouch-shaped case 20.
The shape of a representative battery pack having the pouch-shaped battery with the above-stated construction mounted therein is shown in FIG. 3 and a separated state of the battery pack before being assembled is typically shown in FIG. 4.
Referring to FIGS. 3 and 4, a battery pack 60 includes a rectangular battery 10 having an electrode assembly including cathodes, anodes, and separators received therein together with an electrolyte in a sealed state, a case body 70 having an inner space to receive the battery 10, and an upper cover 80 coupled to the case body 70, in which the battery 10 is received, to seal the battery 10. Between the case body 70 and the battery 10 and between the upper cover 80 and the battery 10 are attached double-sided adhesive tapes 90.
In general, the battery pack 60 with the above-stated construction is assembled by coupling the upper cover 80 to the case body 70, which is made of a plastic material, such as polycarbonate (PC) or acrylonitrile-butadiene-styrene (ABS), using an ultrasonic welding method. The ultrasonic welding method is a method of thermally bonding two surfaces using a frictional heat generated due to vibration based on a high frequency of 20,000 Hz.
However, the demand for a battery pack having a smaller thickness has increased. In recent years, therefore, the thickness of the case body 70 and the upper cover 80 has been reduced to 0.3 to 0.35 mm. As a result, it is difficult to perform die molding and injection molding. In addition, welding strength is reduced with the result that a welding defect rate is increased.
For a battery using a can as a battery case, on the other hand, it is possible to provide proper strength against external impact due to structural characteristics of the can even in a case in which the thickness of the battery case is small. However, the pouch-shaped battery 10 having the structure shown in FIG. 1 has low strength against external impact due to structural characteristics of the pouch-shaped battery 10. For this reason, application of a case having a small thickness to the pouch-shaped battery 10 is limited.
Furthermore, when external impact is applied to the battery pack 60, the battery 10 may move upward and downward in the inner space defined between the case body 70 and the upper cover 80 although the battery 10 is coupled to the case body 70 and the upper cover 80 using the double-sided adhesive tape 90 with the result that a short circuit or a cut off may occur in the battery pack 60.
Consequently, there is a high necessity for a battery pack that can be easily manufactured, has proper strength against external impact while using a case having a small thickness, and exhibits excellent safety against a short circuit or a cut off.
Meanwhile, depending upon kinds of external devices in which secondary batteries are used, the secondary batteries may be used in the form of a single battery or in the form of a battery pack having a plurality of unit cells electrically connected to each other. For example, small-sized devices, such as a mobile phone, can be operated for a predetermined period of time with the power and capacity of one battery. On the other hand, a battery pack having a plurality of batteries mounted therein needs to be used in middle or large-sized devices, such as a laptop computer, a tablet computer, a small-sized personal computer (PC), an electric vehicle, and a hybrid electric vehicle, because high power and large capacity are necessary for the middle or large-sized devices.
A lithium secondary battery, which is one of the secondary batteries, has been widely used due to high power and large capacity thereof.
Various kinds of combustible materials are contained in the lithium secondary battery. As a result, the lithium secondary battery may be heated or explode due to the overcharge of the battery, the overcurrent in the battery, or other external physical impact against the battery. That is, the safety of the lithium secondary battery is very low. For this reason, safety elements, such as a positive temperature coefficient (PTC) element and a protection circuit module (PCM), which are capable of effectively controlling an abnormal state of the lithium secondary battery, such as the overcharge of the lithium secondary battery or the overcurrent in the lithium secondary battery, are loaded on a battery cell of the lithium secondary battery while being connected to a battery cell of the lithium secondary battery.
As described above, small-sized mobile devices use one or several battery cells for each device. On the other hand, middle or large-sized devices, such vehicles, use a middle or large-sized battery module having a plurality of battery cells electrically connected with each other because high power and large capacity are necessary for the middle or large-sized devices. The size and weight of the battery module is directly related to an accommodation space and power of the corresponding middle or large-sized device. For this reason, manufacturers are trying to manufacture small-sized, lightweight battery modules.
In general, a plurality of unit cells is mounted in a cartridge in a state in which the unit cells are connected in series or in parallel to each other and a plurality of cartridges is electrically connected to manufacture a battery pack.
Series connection of unit cells in a conventional high power, large capacity battery pack is typically shown in FIG. 5.
Referring to FIG. 5, each unit cell is configured to have a structure in which a cathode, an anode, and a separator are provided in a case together with an electrolyte in a sealed state and a cathode tab 120 and an anode tab 130 protrude from upper and lower ends of the case. In a case in which a first unit cell 110 is disposed such that a cathode tab 120 is located at the upper end thereof, a second unit cell 111 disposed adjacent to the first unit cell 110 is disposed such that a cathode tab 121 is located at the lower end thereof. Opposite electrodes of the unit cells 110 and 111 are electrically connected to each other via an electrode lead 140 in a state in which the opposite electrodes of the unit cells 110 and 111 are adjacent to each other. A third unit cell 112 is connected in series to the second unit cell 111 in the same manner as connection between the first unit cell 110 and the second unit cell 111. Although not shown in FIG. 5, a first battery group of FIG. 5 including a plurality of unit cells connected in series to each other as described above is connected in parallel to a second battery group including a plurality of unit cells connected in the same manner as in the first battery group.
The battery groups, which are connected in parallel to each other as described above, are mounted in a housing 170 in a state in which the cathode tab 120 of the first unit cell 110 of each of the battery groups is connected to a cathode external terminal 150 and an anode tab 133 of a last unit cell 115 of each of the battery groups is connected to an anode external terminal 160. As shown in FIG. 5, the electrode tabs 120 and 130 are formed in a major axis direction of the unit cell 110. According to circumstances, however, the electrode tabs 120 and 130 may be formed in a minor axis direction of the unit cell 110.
In the battery pack including the unit cells 110, 111, 112, 113, 114, and 115, which are electrically connected to each other, however, the unit cells 110, 111, 112, 113, 114, and 115 have the same size or capacity. In order to reduce weight and thickness of the battery pack in consideration of design of a device to which the battery pack is applied, therefore, it is necessary to reduce the capacity of the battery pack. Alternatively, it is necessary to change the design of the device such that the size of the device can be increased. During a design changing process, however, electrical connection between the unit cells is complicated with the result that it is difficult to manufacture a battery pack satisfying desired conditions.
Consequently, there is a high necessity for a battery pack that is capable of solving the above-mentioned problems.